The growth of portable, personal electronics devices such as cell phones, PDAs, and similar devices, has spurred development of miniaturized cameras and light-sensing components that can be incorporated into these devices. The continuing demand for smaller and more powerful imaging apparatus, coupled with the requirement for low cost, presents a considerable challenge to optical and mechanical design. Low-cost lens assemblies, typically including a number of plastic lens elements, are being used increasingly for these applications.
Although very small plastic lenses can be fabricated inexpensively at high volumes, the handling, alignment, and mounting of these tiny optical components into a lens assembly using multiple components poses significant problems. For mobile imaging applications, for example, two lens elements should be laterally aligned (that is, aligned in the plane normal to the optical axis, where z is the optical axis) to within better than +/−20 microns. There are also tight tolerances with respect to the air space, or longitudinal separation along the optical axis (z axis) between lens elements. Tilt in the two orthogonal directions θx and θy should be controlled to within tens of arc-minutes. Clearly, there is considerable challenge in achieving alignment tolerances in these ranges at low cost when assembling miniature optical components using mass-produced plastic lens elements. Conventional active alignment techniques, such as using point-source microscopy to align centers of curvature individually, prove too complex and costly for high-volume production.
A number of other conventional approaches have been applied to the problem of lens mounting, alignment and centration of lenses, including the use of features formed within a lens barrel or other supporting structure, as described, for example, in U.S. Pat. No. 6,338,819 entitled “High Numerical Aperture Objective Lens Assembly” to Leidig and U.S. Pat. No. 4,488,776 entitled “Plastic Lens Cell” to Skinner. Still other approaches use separate spacing elements to provide proper alignment and air space between optical components. For example, referring to FIG. 1, there is shown a lens mount assembly 10 for mounting multiple lens elements L1, L2, L3 along an optical axis O of a barrel 16. Spacers 12 and surface sags provide proper air space between lens elements L1, L2, and L3 along that optical axis. A retaining ring 13 is then used to hold lens elements L1, L2, and L3 and spacers 12 in place following assembly. Spacers 12, in conjunction with lens flanges, also provide tilt alignment θx and θy. Lateral alignment of lens elements L1, L2, and L3 is accomplished by care in fabrication, controlling tolerance runout of the lenses, the outside diameter of the lenses, and the inside diameter of barrel 16 or other optical mounting structure. However, such approaches increase the overall parts count and assembly complexity and introduce tolerance build-up that can make proper lens alignment difficult, particularly as lens assemblies grow smaller.
Another approach that has been adopted for miniaturized optical systems uses passive component alignment of lens elements to each other, rather than to a barrel or to some other enclosure. Representative examples of optical apparatus using this technique for centration and spacing include:                U.S. Patent Application Publication No. 2003/0184885 entitled “Producing Method of Image Pickup Device” by Tansho et al. discloses an optical unit in which lens elements are stacked against each other to provide centration, with additional spacing elements;        U.S. Patent Application Publication No. 2003/0193605 entitled “Image-Capturing Lens, Image-Capturing Device and Image Capturing Unit” by Yamaguchi discloses a lens barrel wherein a flange is provided on each of one or more stacked lenses, seated against each other to provide both centration and spacing;        U.S. Pat. No. 4,957,341 entitled “Integral Type Lens” to Hasegawa discloses a compound projection lens in which separate lens elements are aligned against each other using a circumferential flange and guide arrangement;        U.S. Pat. No. 4,662,717 entitled “Lens and Lens Holding Devices” to Yamada et al. discloses use of a snap fit for alignment and spacing of adjacent lenses in a lens holding device; and,        U.S. Pat. No. 6,072,634 entitled “Compact Digital Camera Objective with Interdigitated Element Alignment, Stray Light Suppression, and Anti-Aliasing Features” to Broome et al. discloses passive alignment between lens elements in which a tapered fit provides centration and an abutment fit provides proper spacing.        
While each of the above-cited solutions for passive alignment provide some measure of accuracy for centration and spacing, there are inherent problems with each of these approaches that limit their successful application for miniaturized lens assemblies. In particular, each of these proposed solutions exhibits problems due to either or both additive tolerance errors and mechanical overconstraint. The apparatus of both '3605 Yamaguchi and '4885 Tansho et al. disclosures would be particularly prone to lateral centration problems, requiring precision fabrication and assembly of the multiple stacked lens components. For production optical components, in practice, there must necessarily be some finite gap between a lens element and the element that provides its lateral constraint, whether this is provided by a lens barrel or by a structure on an adjacent lens element. Thus, there is some built-in amount of imprecision that is inherent to lateral positioning when using conventional lens mounting techniques as shown in both '3605 Yamaguchi and '4885 Tansho et al. disclosures. The apparatus of both '341 Hasegawa and '717 Yamada et al. patents exhibit overconstraint, limiting the applicability of these approaches to lens assemblies. The apparatus of the '634 Broome et al. patent exhibits both lateral centration and overconstraint problems, with a tapered centration fit of a lens element potentially compromised by an abutment fit for spacing of that same lens element. The '634 Broome et al. solution would thus require highly accurate manufacturing tolerances in order to provide suitable centration alignment and spacing. While the high cost of providing such precision tolerance lens components may be justified for larger, complex optical assemblies, such a design approach would not be compatible with requirements for fabrication of high-volume, low-cost, miniaturized optical assemblies.